DIW-Printed Biomass-Based Radiative Coolers with Phase Change for Extended Thermoelectric Output

Radiative cooling offers a low-energy route for thermal management, but conventional coolers are difficult to match with fluctuating heat loads and suffer from nighttime overcooling. Hence, dynamic radiative coolers were developed through direct ink writing (DIW) of a composite gel ink (MH) comprising thermally treated cellulose nanocrystals (HTCNCs) and a microencapsulated phase change material (MEPCM). A simple thermal treatment converts a low-concentration cellulose nanocrystal (CNC) suspension into a self-supporting HTCNCs gel through localized hydrolysis, promoting the formation of a hydrogen-bonded network. The HTCNCs scaffold provides structural support and mid-infrared emission, whereas MEPCM introduces latent heat storage and release. Accordingly, DIW-printed MH exhibits an average solar reflectance of 96.8% and an average emissivity of 87.0%. Outdoor tests show that the DIW-printed MH cooler lowers the daytime temperature by 2.5 °C and suppresses overcooling at night. When integrated with a thermoelectric generator (TEG), DIW-printed MH effectively extends the duration of the temperature difference between the hot and cold ends. This study provides a strategy for designing dynamic radiative cooling materials and sustainable thermoelectric power generation systems.